DEN(Y)W (standardized modern form)
Morfudd's brother Owein married a woman named Den(y)w (as we shall see, her name is recorded in a vast variety of forms) for whom we have a fair amount of data since her son was Cyndeyrn Garthwys, better known as Saint Kentigern, a prominent early saint of the north of Britain. Her family connections, especially her son, place her in the early-mid 6th century. Trying to pin her name down must begin with assembling the wildly varying evidence. (All citations from Bartrum EWGT unless otherwise noted.)
Textual Sources
The genealogical tract Bonedd y Seint (lineage of the saints) appears in several manuscripts and includes her in the following forms:
Denw Peniarth Ms. 16, third quarter of the 13th c.She is given as Taneu and Tannu in the two manuscripts of Jocelin's Life of Kentigern (ca. 1185) and Thaney in an anonymous life of the saint (ca. 1150) (Forbes 1874). In the Aberdeen Breviary (whose precise date I have not yet been able to track down) she is mentioned twice, as Teneuu and Theneuu (although Bartrum has used different editions of this manuscript in his entries for Denw and Cyndeyrn, and in the latter the name is transcribed as Tenew).Denyw Peniarth Ms. 45, late 13th c.
Dyfuyr Peniarth Ms. 12, ca. 1320 (this may be a substitution of a different name, or influenced by some other word, rather than a simple corruption)
Dinw Hafod Ms. 2, second half of the 14th c.
Dynw Llanstephan Ms. 28 ca. 1475
Dwywai Peniarth Ms. 27 last quarter of the 15th c. & Peniarth Ms. 182 ca. 1510 (this is a substitution of an entirely different name with northern associations, see below)
Dunwl Peniarth Ms. 182, 1514 (this appears to be largely a scribal corruption)
Just to tie things together neatly, Languoreth (q.v.) and her husband Rhydderch Hael feature in the Life of Saint Kentigern.
Linguistic Analysis
Reconciling the forms of this name found in Welsh sources with those in the Scottish biographies might appear to be impossible, but if we assume that the name is of Brythonic origin (which the context of the story implies) then some of the variation can be dealt with. Initial Th in a Latin context is a relatively free variant of T with the same sound quality as the latter. Furthermore, certain aspects of the Brythonic pronunciation of initial voiced stops tended to cause them to be interpreted as voiceless by speakers of other languages, and so an interpretation of underlying d as t seems likely. A final w after a consonant tended to attract epenthetic vowels in Welsh, and would have enough of a vocalic quality in speech that non-Brythonic speakers would tend to interpret it as a vowel, albeit one of uncertain quality. One example from the medieval period is the name Goronwy, which appears in variants such as Gronw and Grono; or the group Cadwy/Cadw/Cado (Bartrum EWGT) found in forms such as Catovii, Cado, Cattw, Gadw, Gadwy. The majority of forms of the name under consideration seem more consistent with a final -nw. Final w after a consonant most commonly derives from a consonantal u (i.e. [w]), less commonly from m. Following the more common possibility, we would expect to find this written with u in the mid 6th century.
This leaves us with the question of the vowel in the first syllable. The earliest Welsh sources seem to agree on e, while the Scottish Lives of Kentigern agree on a. The other northern source, the Aberdeen Breviary, agrees with the Welsh sources, having e. (The later Welsh sources with y and i may be overcompensating for the tendency in some medieval Welsh texts to spell the sound of "y" with e, and may be "correcting" an earlier Denw to Dynw.) I'm inclined to follow the majority here. If this is correct, it's tempting to try to connect the name with the root den- meaning "to attract, allure", but this is pure speculation.
At any rate, we can now suggest reconstructing a mid 6th c. Latinized written form Denua, pronounced something like ['dEn-w@] or, in English syllables, "DEN-weh".
Having already written the above, I discovered that Koch (1997) discusses the name briefly and prioritizes the hagiographical material over the later Welsh genealogical tradition. He equates the name with the common adjective teneu "thin, slender" and suggests a reconstructed pronunciation that, in the 6th century, would be ['tan-ey], or in English syllables something like "TAH-nayw". A corresponding Latinized written form would presumably be something like Taneua. (I could wish that Koch had explicitly discussed the Welsh genealogical material in offering this derivation. Given that the adjective teneu was in common use, and given that the same element was transmitted without significant distortion in the byname Gwrtheneu attributed to Vortigern, the significant changes resulting in the form Den(y)w seem to require a major disruption in the Brythonic transmission of the name.)